3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution, hereinafter referred to as “LTE”) adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme and adopts SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme (e.g. see non-patent literatures 1, 2 and 3).
According to LTE, a radio communication base station apparatus (hereinafter, abbreviated as “base station”) performs communication by allocating resource blocks (RB's) in a system band to a radio communication terminal apparatus (hereinafter, abbreviated as “terminal”) per time unit called “subframe.” Furthermore, the base station transmits control information for notifying results of resource allocation of downlink data and uplink data to the terminal. This control information is transmitted to the terminal using a downlink control channel such as PDCCH (Physical Downlink Control Channel). Here, each PDCCH occupies a resource made up of one or a plurality of continuous CCEs (Control Channel Elements). LTE supports a frequency band having a width of maximum 20 MHz as a system bandwidth.
Furthermore, the PDCCH is transmitted within three initial OFDM symbols of each subframe. Furthermore, the number of OFDM symbols used to transmit PDCCHs can be controlled in subframe units and controlled with CFI (Control Format Indicator) information notified using a PCFICH (Physical Control Format Indicator Channel) transmitted using the first OFDM symbol of each subframe.
Furthermore, the base station simultaneously transmits a plurality of PDCCHs to allocate a plurality of terminals to one subframe. In this case, the base station includes CRC bits masked (or scrambled) with destination terminal IDs to identify the respective PDCCH destination terminals in the PDCCHs and transmits the PDCCHs. The terminal demasks (or descrambles) the CRC bits in a plurality of PDCCHs which may be directed to the terminal with the terminal ID of the terminal and thereby blind-decodes the PDCCHs and detects a PDCCH directed to the terminal.
Furthermore, studies are being carried out on a method of limiting CCEs to be subjected to blind decoding for each terminal for the purpose of reducing the number of times blind decoding is performed at the terminal. This method limits a CCE area to be subjected to blind decoding (hereinafter referred to as “search space”) for each terminal. Thus, each terminal needs to perform blind decoding only on CCEs in the search space allocated to that terminal and can reduce the number of times to perform blind decoding. Here, the search space of each terminal is set using a hash function which is a function for performing randomization with the terminal ID of each terminal.
Furthermore, for the downlink data from the base station to the terminal, the terminal feeds back a response signal indicating the error detection result of the downlink data (hereinafter, referred to as “ACK/NACK signal”) to the base station. The ACK/NACK signal is transmitted to the base station using an uplink control channel such as PUCCH (Physical Uplink Control Channel). Here, to eliminate the necessity for signaling to notify a PUCCH used to transmit the ACK/NACK signal from the base station to each terminal and efficiently use downlink communication resources, the CCE number to which the downlink data is assigned is associated with the resource number of the PUCCH that transmits the ACK/NACK signal corresponding to the downlink data. Each terminal can decide a PUCCH to use to transmit an ACK/NACK signal from the terminal from the CCE to which control information directed to the terminal is mapped. The ACK/NACK signal is a 1-bit signal indicating ACK (no error) or NACK (error present), and is BPSK-modulated and transmitted. Furthermore, the base station can freely set a resource area of the PUCCH to use to transmit the ACK/NACK signal and notifies the start resource number of the resource area of the PUCCH to all terminals located within the cell of the terminal using broadcast information.
Furthermore, transmission power used by the terminal for PUCCH transmission is controlled by a PUCCH transmission power control bit included in the PDCCH.
Furthermore, standardization of 3GPP LTE-Advanced (hereinafter referred to as “LTE-A”) has been started which realizes further speed enhancement of communication compared to LTE. LTE-A is expected to introduce base stations and terminals (hereinafter referred to as “LTE-A terminals”) capable of communicating at a wideband frequency of 40 MHz or above to realize a maximum downlink transmission rate of 1 Gbps or above and a maximum uplink transmission rate of 500 Mbps or above. Furthermore, the LTE-A system is required to accommodate not only LTE-A terminals but also terminals supporting the LTE system (hereinafter referred to as “LTE terminals”).
LTE-A proposes a band aggregation scheme whereby communication is performed by aggregating a plurality of frequency bands to realize communication in a wideband of 40 MHz or above (e.g. see non-patent literature 1). For example, a frequency band having a bandwidth of 20 MHz is assumed to be a basic unit (hereinafter referred to as “component band”). Therefore, LTE-A realizes a system bandwidth of 40 MHz by aggregating two component bands.
Furthermore, according to LTE-A, the base station may notify resource allocation information of each component band to the terminal using a downlink component band of each component band (e.g. non-patent literature 4). For example, a terminal carrying out wideband transmission of 40 MHz (terminal using two component bands) obtains resource allocation information of two component bands by receiving a PDCCH arranged in the downlink component band of each component band.
Furthermore, according to LTE-A, the amounts of data transmission on an uplink and downlink are assumed to be independent of each other. For example, there may be a case where wideband transmission (communication band of 40 MHz) is performed on a downlink and narrowband transmission (communication band of 20 MHz) is performed on an uplink. In this case, the terminal uses two downlink component bands on the downlink and uses only one uplink component band on the uplink. That is, asymmetric component bands are used for the uplink and downlink (e.g. see non-patent literature 5). In this case, both ACK/NACK signals corresponding to downlink data transmitted with the two downlink component bands are transmitted to the base station using ACK/NACK resources arranged on a PUCCH of one uplink component band.
Furthermore, also when the same number of component bands are used for an uplink and downlink, as in the case of using asymmetric component bands as described above, studies are also being carried out on a possibility that a plurality of ACK/NACK signals corresponding to downlink data transmitted in a plurality of downlink component bands may be transmitted from one uplink component band. Here, it is independently set per terminal from which uplink component band of the plurality of uplink component bands an ACK/NACK signal is transmitted.